Not all computers have the same fonts installed. If you want all of your users to see the same font then you can include it like we did when we put the link to shift.css in the head of our index.html file.

unkownhesoka:

why in CSS we use this CSS rule font-family:'shift',sans-serif;

We are using that rule because we want our web page text to use the Shift font, and if that is not available, to fall back to the local computer's sans-serif font.

unkownhesoka:

how I can create my own custom font

That question, I can't help you with, maybe someone who knows more about that will read this.